Acute olfactory response of Culex mosquitoes to a human- and bird-derived attractant.

Abstract

West Nile virus, which is transmitted by Culex mosquitoes while feeding on birds and humans, has emerged as the dominant vector borne disease in North America. We have identified natural compounds from humans and birds, which are detected with extreme sensitivity by olfactory receptor neurons (ORNs) on the antennae of Culex pipiens quinquefasciatus (Cx. quinquefasciatus). One of these semiochemicals, nonanal, dominates the odorant spectrum of pigeons, chickens, and humans from various ethnic backgrounds. We determined the specificity and sensitivity of all ORN types housed in different sensilla types on Cx. quinquefasciatus antennae. Here, we present a comprehensive map of all antennal ORNs coding natural ligands and their dose-response functions. Nonanal is detected by a large array of sensilla and is by far the most potent stimulus; thus, supporting the assumption that Cx. quinquefasciatus can smell humans and birds. Nonanal and CO(2) synergize, thus, leading to significantly higher catches of Culex mosquitoes in traps baited with binary than in those with individual lures.

Representative odorant profiles from humans of various ethnicities. Total ion chromatogram obtained from human volunteers after SPME direct collection and GC-MS analysis. Regardless of ethnic background, all samples comprised of four major constituents whose peaks are labeled in the uppermost trace. Identification was confirmed with synthetic standards.

Odorant profiles from pigeon and chicken obtained by SuperQ and SPME extraction and GC-MS analysis. Nonanal was the major peak detected in chicken and pigeon samples collected by SPME and SuperQ polymer adsorbent. A series of straight chain, saturated and unsaturated aldehydes were detected in minor amounts (Inset).

Analysis of bird- and human-derived odorants by GC-EAD. (A) A live nonbloodfed Cx. quinquefasciatus female was fixed in a truncated pipette tip and secured by modeling clay letting only the eyes and antenna exposed. (Inset) Ground electrode impaled in the eye and two antennae inserted into the recording electrode. Antennal responses to (B) chicken odorants collected on SuperQ polymer and fractionated by gas chromatography and (C) a synthetic mixture containing 50 ng of each human-derived odorants (from left to right: 6-methyl-5-heptene-2-one, nonanal, (E)-2-nonenal, decanal, indole, and geranylacetone). (E)-2-nonenal and indole were added as internal standards. Upper traces (blue) are EAD responses. Upward deflections are due to mechanical disturbances during the GC-EAD runs. Nonanal was the only EAD-active peak in chicken extracts (n = 3). Despite several trials (n = 5), we did not observe EAD response to 6-methyl-5-heptene-2-one. (Scale bar in both the GC-EAD runs, 0.2 mV.)

Captures of Culex mosquitoes in traps baited with nonanal, CO2, or a combination of these two attractants. (A) Nonanal-baited traps caught significantly more mosquitoes than control traps. (B) Synergistic effect of nonanal indicated by significantly higher captures (50% increase) in traps baited with a combination of CO2 and nonanal than those loaded with CO2 alone. (C) Similar results (66% increase) were obtained when the combined lure was compared with individual attractants in a separate location. Treatments followed by the same letters are not significantly different at the 5% level according to Tukey HSD.